The neural basis of perceptual bias and response bias in the Landmark task.

Spatial neglect as a multifaceted syndrome may consist of perceptual/attentional as well as motor/intentional components. The present study investigated the lesion anatomy underlying perceptual and response bias using a manual response Landmark task (Bisiach, Ricci, Lualdi, & Colombo, 1998) in 68 patients with right-hemispheric stroke. The two differential aspects of the neglect syndrome were assessed by measuring response tendencies resulting from underestimations of the length of left line segments and from hypometric movement execution towards contralesional space, respectively. Perceptual and response bias were orthogonal components of task performance in the Landmark task. Perceptual as well as response bias both explained variance in the performance of standard paper-and-pencil neglect tests. While lesions within middle frontal, inferior parietal and parieto-occipital brain regions were related to perceptual bias, subcortical lesions within the caudate were related to response bias in the Landmark task. Our data suggest that perceptual/attentional and motor/intentional aspects of neglect are independent components of the syndrome with differential neural underpinnings in fronto-parietal and subcortical brain regions.